Five imidazolium-type ionic liquids, containing both N1 unsaturated and N3 long alkyl saturated chains as cations and bromide as anion (IL1-IL5), were obtained by conventional and microwave synthesis. Compounds were tested in aqueous 1M H2SO4 as corrosion inhibitors for carbon steel. Weight loss and polarization curves indicated that inhibition efficiency increased with concentration, which turns out to be dependent on alkyl chain size linked to N3 (IL4 > IL3 > IL1 > IL2 > IL5). The relatively high inhibitory properties (88-95%) displayed by IL4 within 25-40 degrees C were ascribed to a chemisorption process that involved the following: the adsorption of protonated imidazolium molecules on both the anodic and cathodic sites, the latter in competition with hydrogen ions to mitigate hydrogen evolution; and also the formation of pi bond with iron by the C=N group from imidazolium ring (this way inhibitor produced more than one center of adsorption action). Surface analysis indicated a considerable reduction of corrosion products after the addition of IL4.